http://forum.dlang.org/thread/okonqhnxzqlqtxijxsfg@forum.dlang.org
TODO: Extend to continuous version; use std.numeric.sumOfLog2s. Ask on StackExchange Computer Science for the correct terminology.
See: http://dlang.org/phobos/std_numeric.html#.sumOfLog2s
TODO: merge with UsageOf
Number of bits required to store a packed instance of T.